What does "80% chance for rain" mean?

July 23, 2008 10:43:06 AM PDT
This percentage, as a forecast for rainfall, is all about statistical probability.

Erica's Answer:

Our computer models predict the weather conditions over the next several days, and compare their predictions to what has happened in the past. For instance, if a day's weather forecast were compared to 10 other days with the same weather conditions in place, and on 8 of those 10 days it rained, then we would have an 80% chance of precipitation.

Then again, some forecasters like to use percentages to tell people how much of the area would get rain. In this case, an 80% chance for rain means that 80% of the Delaware Valley will see rain from that forecast. However, this is "technically" not the correct way to use the percentage in a forecast!

- Erica


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